L10: Maintenance Of Genomic Integrity

Question 1

How does DNA damage occur due to

Answer 1

Error in DNA replication
Spontaneous depurination
Exposure to agents e.g uv light, tobacco, ionising radiation

Question 2

If we do not repair the damaged dna what can occur

Answer 2

Mutation that leads to cancer development

Question 3

What are the 5 major dna repair mechanism

Answer 3

1) direct reversal damage
2) base excision by ROS deamination
3) nucleotide excision repair
4) homologous recombination repair and non homologous end joining
5) dna mismatch repair

Question 4

What are the 3 different types of dna damage that can occur

Answer 4

7-methyl guanine
O-6alkyl guanine
Thymine dimers

Question 5

How does 7-methyl guanine occur

Answer 5

A methyl group adds onto the guanine

Question 6

What does 7-methyl guanine cause a problem with

Answer 6

DNA replication

Question 7

How does a 0-6 alkyl guanine occur

Answer 7

A drug called ethyl methane sulphonate causes guanine to mutate to 0-6 alkyl guanine

Question 8

What is the normal nucleotide paring

Answer 8

A-t

C-g

Question 9

When we get a 0-6 alkyl guanine what nucleotide instead of cytosine (C) does it pair with

Answer 9

Thymine

Question 10

What happen in the dna with 0-6 alkyl guanine during dna replication

Answer 10

Formation of AT pair rather than CG

Question 11

Why does thymine diners occur

Answer 11

Due to uv light exposure

Question 12

What are thymine diners

Answer 12

Formation of covalent bond between adjacent thymine in the dna

Question 13

What does thymine diners cause a problem with

Answer 13

DNA replication

Question 14

Broadly speaking what does dna repair involve

Answer 14

• enzymatic reversal of the damage
  Or
• Removal and replacement of the damage

Question 15

With enzymatic reversal how are uv induced thymine reversed

Answer 15

Via monomeric action by action of visible light and photolyase (enzyme)

Question 16

In enzymatic reversal how are 0-6 alkyl guanine reversed

Answer 16

Alkyl transferase remove the alkyl group on the guanine

Question 17

In enzyme reversal how are strands breaks in sugar phosphate backbone reversed

Answer 17

By ligation

Question 18

What are the substrates of base excision repair

Answer 18

1) spontaneous hydrolyic depurination of dna
2) deamination of cytosine
3) formation of dna adducts

Question 19

Describe what happens in the base excision repair

Answer 19

1) altered dna base is excised from the dna by glycoslyase
2) this leaves an abasic site
3) apurinic site (sugar and phosphate) is removed
4) a new nucleotide is added and ligated via DNA ligase.

Question 20

What are the type of dna that nucleotide excision repair works on

Answer 20

Double stranded DNA (not single stranded dna)

Question 21

What does nucleotide excision repair notice

Answer 21

DNA distortions

Question 22

What can nucleotide excision repair remove

Answer 22

Thymine dimers

Question 23

Describe how nucleotide excision repair occurs when there are thymine dimers

Answer 23

1) thymine dimers are recognised by XPC and XPE
2) we then have the recruitment of TF2H complex that contains XPB and XPD proteins
3) XPA also binds to TF2H complex which verifies the damage
4) XPB and XPD are helicases which cause the unwinding of the dsDNA
5) XPF and XPG are then recruited which are nucleases that cut the sides of the DNA that contain the thymine dimers
6) polymerase then detects the gap and replaces the bases that were removed

Question 24

What is a daughter strand gap repair

Answer 24

When dna that has thymine dimers undergoes dna replication, gaps are left opposite the dimers as the dimers are not recognised.
DNA will gradually repair itself

Question 25

After the repair of the gaps does thymine dimers remain

Answer 25

Yes

Question 26

When are thymine dimers removed

Answer 26

Later by nucleotide excision repair

Question 27

What is xeroderma pigmentosum (XP)

Answer 27

An autosomal receives disorders where patients show excessive sun sensitivity

Question 28

What type of tumours does patients with XP show

Answer 28

Skin tumours

Question 29

Where does the skin tumours appear only at

Answer 29

Areas where skin is exposed to light

Question 30

What defect does XP patients have

Answer 30

Defect in DNA nucleotide exision repair

Question 31

Why do patient with XP have a defect in the dna exision repair

Answer 31

Defeats in the XP genes that are required for NER

Question 32

Name examples of XP genes that XP patients can show

Answer 32

XPA | XPC

Question 33

What defect does the XP variant of xerderma pigmentosum show a defect in

Answer 33

Replication of dna following exposure to uv light i.e daughter strand gap repair

Question 34

What happens in XP when the thymine dimers remix

Answer 34

Further mutation

Question 35

What gene can be further mutated due to unexcised thymine dimers

Answer 35

PTCH1 gene

Question 36

What does PTCH1 mutated gene cause

Answer 36

Basal cell naveous syndrome

Question 37

What are the 2 ways of repairing dna double strand breaks

Answer 37

Homologous recombination repair | Non homologous end joining

Question 38

What molecules does the homologous recombination repair involve

Answer 38

BRCA1 | BRCA2

Question 39

When there is a double stranded dna break which module binds to the end of the breaks to protect it form damage

Answer 39

53BP1

Question 40

What is the role of BRCA1

Answer 40

Removes 53BP1

Question 41

Why does BRCA1 remove 53bp1 from the breaks

Answer 41

To promote resection and recombination to occur

Question 42

What molecule allows the recombination to occur

Answer 42

RAD51

Question 43

How is RAD51 brought to the right position

Answer 43

By BRCA2

Question 44

What other role does BRCA1 have

Answer 44

Cell cycle checkpoints

Question 45

What happens to the a cell that contains BRCA2 mutations

Answer 45

Cannot perform homologous recombination repair

Question 46

How do we treat BRCA2 deficient tumours

Answer 46

Use PARP inhibitor

Question 47

How does PARP inhibitors work

Answer 47

1) inhibit PARP which usually repair single stranded breaks 2) if you inhibit the single stranded break then it will form a double stranded break during DNA replication 3) with PARP inhibitors defect is made worse and leads to cell death

Question 48

When does dna mismatch usually occur

Answer 48

During dna replicaton

Question 49

What are the types of mismatchs that occur in dna

Answer 49

1) Base to base mismatch | 2) insertion, deletion loops due to polymerase slippage during replication

Question 50

What does insertion and deletion loops give rise to

Answer 50

Micosatellites

Question 51

What does micro satellites lead to

Answer 51

Micro satellite instability

Question 52

What is the difference between a base and nucleotide

Answer 52

Nucleotide: a single unit of a polynucleotide chain made of sugar, phosphate group and nitrogenous base Base: the type of nitrogenous base which is either a pyrimidine or purine (A,T,C,G)